Pten-loss Dysregulated Pathways in Prostate Cancer Summary Prostate Cancer (PCa) is the second leading cause of cancer-related deaths (after lung cancer) in American men, and the morbidity and the mortality to PCa are even higher in African American men as compared Caucasians. Molecular mechanisms leading to the initiation and progression of PCa including castration resistant prostate cancer (CRPC) or androgen insensitive prostate cancer (AI-PC) are poorly understood. PTEN and p53 are the two most frequently deleted and/ or mutated genes in a variety of human cancers including advanced PCa. Loss of PTEN leads to the hyperactivation of phosphatidylinositol-3-OH kinase (PI3K) and serine/Thr kinase (Akt/PKB). Pten-deficient mice develop high grade prostatic intraepithelial neoplasia (HGPIN) and invasive adenocarcinoma. We have recently demonstrated in our mouse model that the acute inactivation of Pten unexpectedly elicits cellular senescence, a novel mechanism suppressing cancer progression. Aberrant regulation of oncogenes and tumor suppressors including p19Arf, p53 and p21 proteins have been observed in prostate tumors of Pten-deficient mice. We HYPOTHESIZE that aberrant activation of p19Arf-p53 pathways cooperates with Pten loss to result in prostate cancer progression. We propose to test this hypothesis and to study the molecular mechanisms of regulating Pten-p19Arf-p53 network in prostate cancer using mouse models and cell lines with following Specific Aims: 1. to define the role of p19Arf in prostate cancer progression. 2. to determine the functional roles of p19Arf in the crosstalk of Pten and p53 during tumorigenesis. 3. to address the consequence and relevance of p19Arf inactivation in CRPC growth using Pten/p53 mouse model. Results obtained from this award will provide us valuable insights into novel roles of ARF in prostate cancer progression.